The present invention relates to a rack-and-pinion steering apparatus, which includes pinion teeth provided on the circumferential face of a pinion shaft, and rack teeth provided on the outer face of a rack shaft and engaged with the pinion teeth, wherein the rotation of the pinion shaft connected to a steering member is transmitted to the rack shaft via the engagement portions of the pinion teeth and the rack teeth, and the rack shaft is moved in the axial direction thereof to steer.
Such a rack-and-pinion steering apparatus is widely used as a steering apparatus for vehicles, such as automobiles, and includes a pinion shaft connected to a steering member, such as a steering wheel, pinion teeth being provided on the circumferential face of the pinion shaft, and a rack shaft extended in the right-left direction of a chassis, rack teeth being provided on the outer face of the intermediate portion of the rack shaft along an appropriate length, wherein the rotation of the pinion shaft corresponding to the operation of the steering member by the driver is converted into the movement in the axial direction of the rack shaft to steer wheels to be steered (generally, right and left front wheels) respectively connected to both right and left ends of the rack shaft via tie rods.
In this type of rack-and-pinion steering apparatus, for the purpose of restricting reverse input, such as kickback and flutter, at least one of the tilt angle of the rack tooth (hereafter referred to as rack helix angle) θr with respect to a reference line perpendicular to the axial line of the rack shaft and the tilt angle of the pinion tooth (hereafter referred to as pinion helix angle) θp with respect to the axial line of the pinion shaft is set so as to be larger than the tilt angle of the axial line of the pinion shaft (hereafter referred to as housing intersection angle) θh with respect to the reference line. Hence, in the case that an energizing force exerted by energizing means is set at a large value so that reverse input, such as kickback and flutter, can be reduced, it is difficult to properly convert the movement between the pinion shaft and the rack shaft. As a result, there is a problem of degrading steering wheel returning performance, steering feeling, etc.
For the purpose of solving this problem, Japanese Patent Application Laid-Open No. H6-115440 has disclosed a steering apparatus wherein friction during engagement between the pinion teeth and the rack teeth is reduced by decreasing the pinion helix angle θp so that the movement conversion between the pinion shaft and the rack shaft can be carried out properly and efficiently. With this configuration, an energizing force exerted to the rack shaft by energizing means can be set at a larger value, a minute relative displacement between the pinion shaft and the rack shaft can be absorbed effectively by the friction during engagement between the rack teeth and the pinion teeth, and reverse input, such as kickback and flutter, can be reduced while steering wheel returning performance, steering feeling, etc. are prevented from becoming deteriorated.
However, in the above-mentioned steering apparatus, in the case that the diameter of the pinion shaft is set constant, stroke ratio is reduced by decreasing the pinion helix angle θp. Hence, the range in which the pinion helix angle θp can be decreased cannot be increased in design. In addition, the diameter of the pinion shaft is restricted because of the space around the steering gear. Therefore, the practical design range of the steering apparatus is limited to a very narrow range, and there is a problem of having almost no degree of design freedom.